The active maintenance of a constant environment is known as homeostasis, and it is a critical attribute of cells and of life itself. Chemical reactions and protein folding, for example, are carried out efficiently only within a narrow range of conditions. How does the plasma membrane maintain homeostasis? The answer is that it is selectively permeable (or semipermeable). This means that the plasma membrane lets some molecules in and out freely; it lets others in and out only under certain conditions; and it prevents other molecules from passing through at all.

The membrane’s ability to act as a selective barrier is the result of the combination of lipids and embedded proteins of which it is composed. The hydrophobic interior of the lipid bilayer prevents ions as well as charged or polar molecules from diffusing freely across the plasma membrane. Furthermore, many macromolecules such as proteins and polysaccharides are too large to cross the plasma membrane on their own. By contrast, gases, lipids, and small polar molecules can freely move across the lipid bilayer. Protein transporters in the membrane allow the export and import of molecules, including certain ions and nutrients, that cannot cross the cell membrane on their own.

The identity and abundance of these membrane-associated proteins vary among cell types, reflecting the specific functions of different cells. For example, cells in your gut contain membrane transporters that specialize in the uptake of glucose, whereas nerve cells have different types of ion channel that are involved in electrical signaling.